The lower portion of the door frame is separated from the rest of the door frame and attached again with hinges. In order to come through the door the user folds out the door frame portion that sits on hinges, which widens the doorway.
How to make
Open the door and use a saw to cut a horizontal track in the door frame above and below at the appropriate height. Then saw a vertical track and remove the door frame portion, attach hinges to it and screw the door frame portion back in place.
This tip is from 2008